Feature: Add the dashed style. (#594)

CHANGELOG.md

examples/dom/border_style.cpp

examples/dom/separator_style.cpp

include/ftxui/dom/elements.hpp

src/ftxui/dom/border.cpp

src/ftxui/dom/separator.cpp

src/ftxui/dom/separator_test.cpp

src/ftxui/dom/table.cpp

src/ftxui/screen/screen.cpp